THE PHYSIOLOGICAL ANALYSIS OF BEHAVIOR 157 



the light thus constantly playing upon the photosensitive area 

 leaves no room to doubt that the organism is as surely in a 

 condition of constant stimulation as is Paramecium when exposed 

 to a constant galvanic current. Its orientation is forced. 



Can the relation between the photic energy applied to the 

 organism and the reaction of the latter be expressed in quanti-. 

 tative terms ? Though this has not yet been accomplished for 

 Euglena, experiments have shown that the relation can be ex- 

 pressed for several other organisms in terms of the law origin- 

 ally offered by Bunsen and Roscoe as a formulation of the effect 

 produced by light upon silver chloride. According to this law, 

 stated in more general terms than those originally employed, 

 the effect of the light radiation is proportional to the product 

 of the intensity of the light and its duration, that is, to the 

 total amount of photic energy supplied. 



The first application of the Bunsen-Roscoe law to biological 

 phenomena appears to have been made by the plant physiolo- 

 gist Blaauw. The results of his investigations on oat seedlings 

 (Avena) and a fungus (Phycomyces) published in 1909, are in 

 striking conformity with the theoretical expectation. It will 

 not be possible here to consider either the details of his experi- 

 ments or their technic. It must suffice to say that in a series 

 of twenty-six experiments on Avena, he varied the duration of 

 the light from 1-1000 of a second to 43 hours, and its intensity 

 in inverse ratio so that the energy delivered should be a constant 

 quantity; and that in spite of this enormous range in the values 

 of these factors, the extreme experimental variation from the 

 average among the entire twenty-six measurements, was but 

 28 per cent. When it is remembered that the response of an 

 organism to a given stimulus is necessarily complicated by the 

 influence of various other stimuli whose effects may not even 

 be recognized, much less accurately determined, I believe we 

 shall be willing to admit that Blaauw' s experimental results 

 demonstrate the applicability of the Bunsen-Roscoe law to the 

 phototropism of plants. 



Later experiments have shown the applicability of this law 

 to the phototropism of animals also. Loeb has obtained evidence 

 of its existence in a hybroid {Eudendrium). Ewald has found it 

 to apply to the eye movements of Daphnia under appropriate 



